Method of quenching metals in an aqueous silica sol



United, States Patent John J. Cox, Jr., Wilmington, Del., assignor to 1*..1. du

' Pont de Nemours and Company, Wilmington, Del.,

a corporation of Delaware No Drawing. Application July 18, 1957 Serial No. 672,554

4 Claims. (Cl. 14818) This invention relates to processes for quenching metal articles. It is more particularly directed to processes for quenching such articles in silica sols.

It has heretofore been the practice to quench steel and similar metal articles by heating to an appropriate temperature and then immersing the steel in a liquid such as oil or water which removes heat at a desired rate. Water has been none too satisfactory for many purposes because of the rapid rate of heat removal and while oils have found wide application they are hazardous because of flammability and because of the noxious vapors which arise from them. Oils moreover do not always give the a desired rate of heat transfer for a particular application.

Processes of the invention can be used for quenching any metal and will generally be found most applicable to the quenching of metals and alloys which are subject to a phase transformation on cooling. Thus in quenching steel it is a purpose to heat a steel to at least partly an 'ai stenitic state and by quenching to produce a martensitic structure. Hardening of steel by quenching implies the formation of martensite to a desired extent in accordance with customary practices.

Metals which can be advantageously quenched according to the invention are those which have heretofore been quenched in the art. These include plain carbon steels, low alloy steels, high alloy steels, hardenable stainless steels such as type 430, aluminum bronzes, and titanium alloys.

Articles formed of such metal alloys can be heated according to the invention in precisely the same way as in the prior art. Thus they are heated to a temperature above their transformation point and then quenched by immersion in a silica sol.

When reference is made herein to immersing the article in a quenching liquid it will be understood that various means for effecting contact customarily used in the art can be employed. Thus occasionally the liquid is sprayed or flowed upon the article to be treated.

Using a silica sol according to processes of the invention, the rate of heat transfer can be controlled by adjusting the concentration of silica in the sol. Thus by proper selection of a silica content one can obtain a heat transfer approximately equivalent to each of the various liquids heretofore used as quenching media ranging from, say, water at the one extreme to heavy quenching oils at the other. Moreover by the use of relatively concentrated silica sols it is possible to obtain rates of heat transfer which are even slower than those which can be obtained using quenching liquids customarily available.

According to the present invention any silica sol can be used.

Particularly advantageous sols are those of the Bechtold and Snyder Patent 2,574,902. Ordinarily these sols contain about percent of SiO as they are sold in commerce and they can be diluted to whatever extent desired for use in processes of the invention. The particle size of such sols will ordinarily be around 15 millimicrons in See diameter though sols can be made according to the processes of that patent with particles ranging in diameter up to the limits of colloidal dimensions. The SiO :Na O weight ratio of the sols will range upwardly from 60:1.

' Other sols which can be used are those of the Rule Patent 2,577,485. Sols of this patent ordinarily have a relatively high mol ratio of SiO :Na O ranging upwardly from :1 to 500:1. These will be particularly advantageous where the presence of any large amount of alkali is undesirable.

Another type of sols which can be used are those of Alexander 2,750,345. These are sols in which the particles are discrete and range from about 5 to 8 millimicrons in diameter.

Sols of the Bird Patent 2,244,325 and Voorhees Patent 2,457,971 can be used if a relatively dilute sol is suitable for the particular quenching operation. Other prior art sols described in the Bechtold and Snyder Patent 2,564,902 can also be employed.

Still other sols include those which can be produced by dispersing finely divided silica in Water such as that prepared by burning hydrogen with air saturated with silicon tetrachloride. Such sols are disclosed in Reik US. Patent 2,428,178 and in Broughton US. Patent 2,535,036.

Still other silica sols which can be employed in processes of the invention are those shown in White 2,285,477; Marshall 2,356,774; White 2,375,738; Trail 2,572,578; Trail 2,573,743; and Legal, Jr., 2,724,701.

The silica sols employed can vary in ratio from an SiO zNa O mol ratio of about 60:1 or even lower up to 500:1 or even higher. Ordinarily the silica sols of commerce can be used at the ratio at which they are sold, though the ratio can be adjusted by addition of alkali or by its removal.

The quantity of silica to be present in the silica sol employed as a quenching liquid according to the invention can be widely varied. Thus from a fraction of a percent to the maximum silica content which can be obtained in a sol stable for the duration of a quenching operation can be advantageously used. The numerous silica sols above described are shown in various concentrations and the concentrations there shown can be used ranging from a fraction of a percent up to 45 or even 60 percent or more of SiO By using a selected small amount of silica in the sols to be used one can obtain a correspondingly small effect and can obtain relatively rapid quenching. This can be adjusted with some precision depending upon the particular alloy being quenched and upon the character of the desired final product.

Selected larger amounts can be used to give a desired rate of transfer of heat depending upon the specific alloy and upon the shape of the article and the precise character of the finished product desired, all in accordance with considerations well understood in the quenching art.

The essential feature of the present invention is the provision of aqueous quenching liquids and particularly of aqueous quenching liquids which can have any of a wide range of selected heat transfer abilities.

'When articles are treated according to processes of the invention a film or coating is ordinarily formed upon their surfaces. The thickness of this film depends upon a number of factors including the concentration of the silica sol and the size of the article being treated. It also depends upon the SiO zNa o ratio, the lower the ratio the less the tendency to form a film.

The film which forms can be removed by mechanical means such as tumbling of the articles, brushing, caustic cleaning or other chemical cleaning means, or by sand blasting. In some instances it will be desirable to leave A metal article of 4140 steel is heated to 1550 F. This is then immersed in an aqueous silica sol containing 30 percent SiO in the form of particles 17 millimicrons in diameter. The sol is prepared according to the Bechtold and Snyder patent above cited. The article produced by quenching is markedly softer than an equivalent article quenched in water.

Bars of 4140 alloy prepared according to A.S.T.M. test A255-48T, commonly called the Jominy test, and quenched according to that test with water and with a' 30 percent sol as in this example showed a marked difierence in the depth of hardening and in the shape of the Jorniny curve. The bar quenched with silica sol was much softer at the quenched end than a bar quenched with water and was softer throughout the length than one quenched with water. A marked decrease in cooling rate is evident from the Jominy curve.

Example 2 Example 3 Articles of 4130 steel, 1% inch in diameter, wereheated to 1550 F., preliminary to quenching. These articles were quenched in silica sols containing 1 percent, 5 percent, percent, 20 percent, and 32 percent silica. The

softness of the articles quenched in these sols increased continuously as the concentration of silica in the sols was increased. Furthermore, the uniformity of the hardness increased as the concentration of silica in the sol was increased. The article quenched in the 32 percent sol had practically uniform hardness across its cross section compared to a similar article quenched in pure water, which had a soft center and a harder surface.

Silica sols like those of the above examples but containing larger or smaller amounts of SiO can be used in the same way with the alloys shown and with others. Thus silica sol quenching liquids containing 1 percent, 5 percent, 10 percent, 18 percent, percent, or even percent of Si0 can be used depending upon the shape and size of articles treated, the precise alloy employed, the magnitude of the effect desired, as well as the quenching rate wished.

I claim:

1. In a process for quenching metals, the step comprising immersing a heated metal article into an aqueous silica sol.

2. In a process for quenching steel and steel alloys, the step comprising immersing a steel article heated at least partially to the austenitic state into an aqueous silica sol of particles of 5 to millimicrons diameter.

3. In a process for quenching metals, the step comprising immersing a heated metal article into an aqueous silica $01, the silica content being'high enough to give the rate of quenching desired.

4. In a process for heat treating steel, thesteps of heating the metal to the austenitizing temperature and then quenching the metal in an aqueous silica sol containing not more than 32% SiO in the term of particles 17 millimiorons in average diameter.

References Cited in the file of this patent The Quenching of Steels, by H. J. French, copyright 1930, by the ASST, page 9. 

